I haven't experienced it myself, but the consensus at TWC appears to be that reinforceing armies are bugged to the point where they often proceed to a corner and huddle.

I only had one battle with a reinforcing army thus far, and I was able to smash our opponent before they arrived, thus I wasn't watching what they were doing.

Reports indicate armies assisting the player or the AI will occasionally not fight. Only becomes REALLY problematic in sieges with the battle timer turned off.